An aluminum alloy heat exchanger which is formed of aluminum alloy and has a good lightweight and good thermal conduction is widely used, for example, in a condenser and an evaporator for a room air-conditioner; a condenser, an evaporator, a radiator, a heater, an intercooler, an oil cooler, and the like for a vehicle. The aluminum alloy heat exchanger is normally configured by bonding a fin material and a tube material (constituent of a working fluid passage).
As a bonding method of an aluminum alloy material, various methods have been known. However, a brazing method is used much among the methods. The reason of using the brazing method much is because an advantage, for example, in that strong bonding for a short period of time is obtained without melting of a matrix is considered. As a method of manufacturing the aluminum alloy heat exchanger using the brazing method, the following methods have been known (PTL 1 to PTL 3): a method of using a brazing sheet obtained by cladding a brazing filler material formed of an Al—Si alloy; a method of using an extrusion material on which brazing filler material powder is applied; a method in which materials are combined, and then another brazing filler material is applied on a portion at which bonding is required; and the like. Details of the clad brazing sheet or the brazing filler material powder are described in “3.2 wax and brazing sheet” of NPL 1.
In brazing of the fin material and the tube material, when a single-layer fin material is used, a method of using a brazing sheet obtained by cladding a brazing filler material on the tube material, or a method of individually coating the tube material with a Si powder, a Si-containing wax, or a Si-containing flux is employed. When a single-layer tube material is used, a method of using a brazing sheet obtained by cladding a brazing filler material on the fin material is employed.
In this manner, a material obtained by forming a construction derived from a wax on a surface of at least one of the fin material and the tube material is used in manufacturing of a heat exchanger using brazing. For example, in a heat exchanger manufactured by using a single-layer fin material, a portion of a surface of a tube, at which a eutectic structure derived from a wax exists, appears. This portion serves as a cathode site, accelerates progress of corrosion in a tube, and thus leakage of a refrigerant occurs early.
As a heat exchanger used under a high corrosion environment, a heat exchanger which prevents a leakage of a refrigerant by using a clad fin material in such a manner that a eutectic structure is not formed on a surface of a tube by using a wax is considered.
PTL 4 discloses a method of using a single-layer brazing sheet instead of the above-described brazing sheet of a clad material, in order to omit a process of manufacturing a brazing sheet or a process of manufacturing and applying a brazing filler material powder. In this method, it has been proposed that a single-layer brazing sheet for a heat exchanger is used in a tube material and a tank member of a heat exchanger.
PTL 5 discloses a bonding method that obtains well bonding and causes deformation to hardly occur by controlling an alloy composition, a temperature in bonding, pressing, a surface status, and the like in a method of manufacturing a bonding object by using a single-layer aluminum alloy material.
PTL 6 discloses that high corrosion-resistant bonding object is obtained by controlling components of one aluminum alloy material and a pitting potential difference in a structure of the one aluminum alloy material in a bonding object bonded without using a bonding member.
In a case of a heat exchanger obtained by combining a tube material in which a wax is not included on a surface, and a clad fin material, a tube may obtain high corrosion resistance, but corrosion in a fin may be in progress, and thus sufficient cooling performance may not be obtained early. Particularly, there is a problem of often occurrence of corrosion that one thin film remains on a surface of the fin and a core portion on the inside is dissolved (referred to as “hollow corrosion” below).
Such hollow corrosion occurs due to a fin of a heat exchanger having a structure as in a schematic diagram illustrated in FIG. 8(a). That is, the heat exchanger has a layer in which an Al matrix (region A) and an Al matrix (region B) are provided. In the Al matrix (region A), a fine Al—Fe—Mn—Si based intermetallic compound is dispersed at a core portion. In the Al matrix (region B), the fine Al—Fe—Mn—Si based intermetallic compound does not exist on a surface. High-concentration Si is provided at a grain boundary of the core portion by surrounding matrices. In this structure, corrosion occurs easiest at the grain boundary having a high-concentration Si portion which is a strong cathode. Thus, intergranular corrosion occurs at an early stage (FIG. 8(b)). The next easiest corrosion-occurrence portion is the region A of the Al matrix in which the fine Al—Fe—Mn—Si based intermetallic compound is dispersed. This is because the fine Al—Fe—Mn—Si based intermetallic compound dispersed in the Al matrix acts as a cathode and the surrounding Al matrices are dissolved. For this reason, corrosion may occur easier in the region A than in the layer (region B) of a surface which is not the portion acting as the cathode, and of inside corrosion may be in progress (FIG. 8 (c)). In a case of such a state, there is a problem that even though a shape of the fin is ensured seemingly, heat performance is greatly degraded by a hollow portion which occurs due to hollow corrosion.
In order to prevent occurrence of hollow corrosion in the fin, a method of replacing a material of a member as disclosed in PTL 4 and PTL 6 with a fin material is considered. However, even though the material disclosed in these literatures is used simply as the fin material, holding of a shape of the fin in the heat exchanger is impossible and buckling occurs in bonding. Thus, there is a problem that manufacturing of a heat exchanger by using these is impossible.